Molecular oxygen, through the action of its reactive metabolites, is capable of causing both acute and chronic tissue injury. Growth and maintenance of cells under normoxia results in a small but continuous production of partially reduced oxygen radicals, including superoxide and H2O2. The powerful cytotoxic agent, HO., is believed to be formed by a non-enzymatic but iron-catalyzed reaction between superoxide and H2O2. Superoxide dismutase, one of the major intrinsic defense systems of the cell against oxidative stress, exerts its protective action by removing superoxide, one of the substrates for HO. formation. Several diseases related to hypoxia and hyperoxia are ascribed to a dramatic increase of superoxide generation, to such an extent that the level of superoxide dismutase activity may no longer be sufficient to remove all superoxide. Superoxide dismutase exists in eukaryotic cells in two different forms. The Cu/Zn SOD in the cytosol and the MnSOD in the mitochondrial matrix. In response to oxidative stress, the MnSOD of some cell types increases in content. In other cell types, however, no inductive response occurs. Furthermore the capacity to respond to oxidative stress with increased mnSOD can be lost upon aging. It appears that the increase in MnSOD triggered by some form of oxidative stress provides enhanced intrinsic protection against oxidative damage. Previous studies with yeast indicate that oxygen, heme and catabolite repression each affect MnSOD biosynthesis. The experimental data do not preclude the possibility that these agents each exert their effect via alteration of respiratory chain activity and concomittant superoxide generation by the mitochondrion. No reliable method is available to determine superoxide generation in intact mitochondria due to the presence of mitochondrial MnSOD. The recently constructed MnSOD-deficient mutant grown in this laboratory will provide the means to study the superoxide generation in intact mitochondria. It will then be possible to examine whether factor affecting the MnSOD biosynthesis exert their effect by alteration of the superoxide generation. Upon establishing the factor(s) that regulate the MnSOD biosynthesis a series of experiments will be conducted to determine whether inducing factors operate at the transcriptional, translational or post-translational level. This will be accomplished by comparing the concentration of MnSOD mRNA, using the recently isolated MnSOD gene, the rate of in vivo protein synthesis and degradation and steady-state protein concentration.